The diamond form of carbon possesses many desirable physical properties such as hardness, chemical inertness, infrared transparency, and excellent heat conductivity coupled with very high electrical resistivity. Consequently, diamond is a material with many important technological applications such as in optical devices, semiconductors, heat sinks, abrasives, tool coating, etc. It can also be used in film form as a high-grade, radiation-resistant, high-temperature semiconductor with obvious applications in many technologies. Thus, there is considerable incentive to find a practical way to produce diamond films by CVD which have a smooth growth surface.
Various methods are known for forming diamond films or coatings. One such method is disclosed in U.S. Pat. No. 4,707,384. Another method is disclosed by E. V. Spitsyn et al., "Vapor Growth of Diamond on Diamond and Other Surfaces", J. of Crystal Growth 52, pp. 219-226 (1981). Additional methods are disclosed in U.S. Pat. Nos. 4,486,286, 4,504,519, 4,645,977 and 4,707,384. One of the methods developed for deposition of synthetic diamond films, viz., the low pressure chemical vapor deposition (CVD) method, has found favor in the literature recently.
One of these techniques involves the use of a dilute mixture of hydrocarbon gas (typically methane) and hydrogen, wherein the hydrocarbon content usually is varied from about 0.1% to 2.5% of the total volumetric flow. The gas is introduced via a quartz tube located just above a hot tungsten filament which is electrically heated to a temperature ranging from between about 1750.degree. C. to 2400.degree. C. The gas mixture disassociates at the filament surface, and diamonds are condensed onto a heated substrate placed just below the hot tungsten filament. The substrate is heated to a temperature in the region of about 500.degree. C. to 1100.degree. C.
The second technique involves the imposition of a plasma discharge to the foregoing filament process. The plasma discharge serves to increase the nucleation density and growth rate and is believed to enhance formation of diamond in the form of a film as opposed to discrete diamond particles. Of the plasma systems that have been utilize in this area, there are three basic systems: one is a microwave plasma system, the second is an RF (inductively or capacitively coupled) plasma system, and the third is a d.c. plasma system. The RF and microwave plasma systems utilize relatively complex and expensive equipment which usually requires complex tuning or matching networks to electrically couple electrical energy to the generated plasma. Additionally, the diamond growth rate offered by these two systems can be quite modest.
A third method in use is direct deposit from acetylene as a hydrocarbon-rich oxyacetylene flame. In this technique, conducted at atmospheric pressure, a specific part of the flame is played on a substrate on which diamond grows at rates as high as 100 microns/hour or more. See Y. Matsui et al., Japan Journal of Applied Physics, 101, 28, p. 178 (1989).
In general, processes for the chemical vapor deposition of diamond involve selection of operating parameters such as the selection of a precursor gas and diluent gases, the mixture proportions of the gases, gas temperature and pressure, the substrate temperature, and means of gas activation. These parameters are adjusted to provide diamond nucleation and growth on a substrate. Mixture proportions and conditions must provide atomic hydrogen to stabilize the surface of the diamond film and preferably minimize the deposition of graphite. Codeposition of graphite is more evident if the hydrocarbon (methane) concentration is increased above about 3%.
The known CVD techniques provide diamond films with a rough chaotic surface typical of a polynucleated crystalline material and exhibit a Raman line at 1332 cm.sup.-1. However, some of the end-use applications for diamond films, e.g., high velocity water jet, particulate solid abrasive and air nozzle, e.g., for sand blasting, and a particulate solid abrasive and water nozzle require that one or both of the surfaces (faces) of the diamond film be smooth. Although a diamond film only one side of which is smooth can be produced by depositing the diamond film by a CVD process on a polished substrate, and thereafter separating the substrate, e.g., by acid etching, producing a self-supporting diamond film by CVD which is smooth on both sides or a diamond film on a substrate whose growth surface is smooth has not heretofore been accomplished because the active growth surface of a diamond film deposited by CVD in a conventional manner has a rough faceted crystalline surface. As the thickness of a CVD diamond film increases, the crystal size also increases as well as the size of the crystal facets on the growing face. Consequently, the roughness of the growing surface of the diamond film increases as the thickness of the film increases.
Although post-deposit abrasive polishing of the growth surface of a CVD diamond film is theoretically possible, it would be very difficult and prohibitively expensive to do so.
It is desirable to produce thick CVD diamond films with an exposed (growth) surface which is smooth.